We report the experimental observation of Landau quantization of molecular beam epitaxy grown
Sb2Te3 thin films by a low-temperature scanning tunneling microscope. Different from all the reported
systems, the Landau quantization in a Sb2Te3 topological insulator is not sensitive to the intrinsic
substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as
a 2D massless Dirac fermion system is achieved. We demonstrate that four quintuple layers are the
thickness limit for a Sb2Te3 thin film being a 3D topological insulator. The mechanism of the Landaulevel
broadening is discussed in terms of enhanced quasiparticle lifetime.